The present invention relates to a laminated chromium plating layers and a method for producing the same wherein the lamination of chromium plating is applied to sliding components used in internal combustion engine, such as the outer peripheral surface or the end face of a piston ring, the inner peripheral sliding surface of a cylinder liner, the sliding surface of a rocker arm, the outer peripheral surface of a cam lobe of a cam shaft, or a journal portion.
Recently, demand for internal combustion engine with highly efficient fuel consumption rate, high output, and the like has strengthened. In association with this, the load on various components in the internal combustion engine, such as on piston rings, has continually increased.
Conventionally, to deal with improvements in performance, such as in wear resistances of piston rings for internal combustion engine and in the thermal seizure resistance of cylinder liners, the friction surface of the piston rings and the cylinders are plated with high hardness chromium, which has superior wear resistance and thermal seizure resistance.
Conventionally, chromium plating is formed continuously without any layer boundaries. Moreover, the chromium plating portion is formed with a fixed hardness throughout. During frictional sliding operation, a chromium plating layer having a configuration of a single layer receives a strong force from the chromium plating surface in the sliding direction. Also, at a combustion stage in the engine, the layer receives a strong impact force. However, conventional single layer chromium plating formed continuously has high hardness, so that a toughness of the layer is insufficient and the layer has low anti-breakage limits. Generally, "toughness" implies resistance against breakage due to the application of external force. Low toughness implies high tensile strength, and high toughness implies high compression strength without longer elongation.
Japanese Patent Application Publication (Kokai) No. HEI-10-53881 discloses a laminated chromium plating layers which is an improvement on the conventional single chromium plating layer in order to improve wear resistance. According to the disclosed technique, a thin high hardness chromium plating layer is repeatedly precipitated, to form a lamination of chromium plating layers having many minute cracks that is independent in the direction of lamination thickness. That is, the minute cracks become closed cells when the upper plating layer is formed on the lower plating layer. By this, the amount of oil retained in the minute cracks in the chromium plating increases. More specifically, even if the upper plating layer is frictionally worn, the lower plating layer can newly provide oil retaining recesses at the minute crack portions to maintain oil retainability. Thus, wear resistance can be improved.
However, in a portion of internal combustion engine, the above-described conventional high hardness chromium plating can no longer sufficiently deal with required performance in terms of wear resistance and fatigue strength. That is, even though the high hardness chromium plating has sufficient wear resistance because of high hardness of the plating layer and because of the oil retaining function of the minute cracks. However, high hardness of the plating layer increases a modulus of elasticity (becomes excessively rigid). At the same time, the minute cracks provide the notch effect, so that repeated load during engine rotation invites the cracks to enlarge, ending in a danger of breakage. The present fatigue strength is insufficient for dealing with this problem.
Also, there is a problem with the laminated chromium plating layers that are formed by repeatedly precipitating thin high hardness chromium plating layer while forming independent minute cracks in the direction of layers, in that it has as large a modulus of elasticity as ever since each layer is formed of the high hardness chromium plating film. Thus, such conventional layers also provide low fatigue strength.
Japanese Patent No. 2602499 discloses a chromium plating layer in which the minute cracks are formed, and solid high hardness particles are supported in gaps of the minute cracks to improve wear resistance of the chromium plating layer. The chromium plating layer containing high hardness particles may increase frictional wearing of the opponent sliding layer. Further, the chromium plating layer containing high hardness particles must have minute cracks with width broadened to the diameter of the high hardness particles. Therefore, such arrangement increases notch effect to further lower the fatigue resistance, even though wear resistance can be increased.
In order to solve these problems, it is presently strongly desired that a film material capable of providing a high function improved over the conventional chromium plating be obtained at a low price.